The disclosure relates to a piston unit and to a hydrostatic radial piston machine having a piston unit of this kind.
A hydrostatic radial piston machine of the type in question, which is known from DE 40 37 455 C1 for example, has a rotor which is embodied as a cylinder block and in which a multiplicity of piston units are guided for movement in the radial direction. Each of these piston units has a piston, which is guided in a cylinder bore of the cylinder block and the end portion of which, which protrudes from the cylinder block in the radial direction, is supported via a rolling element on a cam ring forming part of the housing of the radial piston machine, with the result that the pistons perform a stroke in accordance with a cam curve on this cam ring.
It has been found that conventional hydrostatic radial piston machines of this kind are subject to a not inconsiderable friction in the region of the sliding surfaces, the effect of such friction being, in particular, to reduce the required starting torque when the radial piston machine is used as a radial piston motor and thus to impair the efficiency thereof.
To overcome this disadvantage, a pocket supplied with a pressure medium can be provided in the region of contact between the pistons and the rolling element, thus ensuring that the rolling element is supported hydrostatically and hence that the friction between the pistons and the rolling element is significantly reduced. Such a solution is shown in DE 10 2010 032 058 A1, for example. In order to avoid tilting of the piston within the cylinder bore, the piston can be designed as a stepped piston, ensuring that said piston is guided in the cylinder block over a large guidance length and nevertheless permitting a large effective piston cross section.
In the abovementioned publication, there is a proposal to avoid leakage from the region of the hydrostatic bearing by making a holding portion of the piston, along which the rolling element is held, elastic, thus ensuring that the holding portion as it were “hugs” the rolling element and hence reduces a gap that would allow leakage.
Despite these design measures, it has been found that the starting torque of such radial piston machines and piston units when new, i.e. before the radial piston machine has been running, is poorer than in the case of a radial piston machine which has already been in service for a number of hours.
In contrast, it is the underlying object of the disclosure to provide a piston unit and a radial piston machine embodied with a piston unit of this kind in which the starting torque when new is improved over conventional solutions.